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Noritic Anorthosite Bodies in the Sierra Nevada Batholith

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Noritic Anorthosite Bodies in the Sierra Nevada Batholith MINERALOGICAL SOCIETY OF AMERICA, SPECIAL PAPER 1, 1963 INTERNATIONALMINERALOGICAL ASSOCIATION,PAPERS, THIRD GENERAL MEETING NORITIC ANORTHOSITE BODIES IN THE SIERRA NEVADA BATHOLITH ALDEN A. LOOMISl Department of Geology, Stanford University, Stanford, California ABSTRACT A group of small noritic plutons were intruded prior to the immediately surrounding granitic rocks in a part of the composite Sierra Nevada batholith near Lake Tahoe. Iron was more strongly concentrated in the late fluids of individual bodies than during the intrusive sequence as a whole. Pyroxenes are more ferrous in rocks late in the sequence, although most of the iron is in late magnetite which replaces pyroxenes. Both Willow Lake type and normal cumulative layering are present. Cumulative layering is rare; Willi ow Lake layering is common and was formed early in individual bodies. Willow Lake layers require a compositional uniqueness providing high ionic mobility to explain observed relations. The Sierran norites differ from those in large stratiform plutons in that (1) the average bulk composition is neritic anorthosite in which typical rocks contain over 20% AbO" and (2) differentiation of both orthopyroxene and plagioclase produced a smooth progressive sequence of mineral compositions. A plot of modal An vs. En for all the rocks in the sequence from early Willow Lake-type layers to late no rite dikes defines a smooth non-linear trend from Anss-En76 to An,,-En54. TLe ratio An/ Ab decreased faster than En/Of until the assemblage Anso-En65 was reached and En/Of began to decrease more rapidly. Similar plots for large stratiform bodies show too much scatter to define single curves. Temperature gradients of 40°-50° C. within the magma chambers and variable convection patterns could account for the observed variations. INTRODUCTION hornblende. The largest quartz diorite body contains A group of consanguineous intrusions crops out local Willow Lake type combed layers as rims aroun \ around Eagle Lake, just southwest of Emerald Bay, inclusions and as thin sheets on internal intrusive Lake Tahoe in the fifteen-minute Fallen Leaf Lake contacts. The present volume of the earlier noritic quadrangle, California. The rock types in the se- rocks is only about 10 per cent of that of the later quence, from oldest to youngest, are (1) noritic quartz diorites. anorthosite and leuconorite, (2) hypersthene diorite, (3) hornblende quartz diorite and (4) hornblende- COMPOSITION AND TEXTURES biotite quartz diorite with minor granodiorite. The The rocks of the noritic sequence are much more two oldest groups of rocks will be called the Eagle feldspathic than the more common early gabbros and Lake sequence; their generalized relations with diorites of this part of the Sierra Nevada (Loomis, other rocks is shown in Fig. 1. The noritic anortho- 1961). Compton (1961) and Taubeneck and Polder- site-hypersthene diorite sequence comprises many vaart (1960) have reported similar noritic rocks from small bodies with complex intrusive relations among the northern Sierra Nevada and the Wallowa batho- one another. The bodies are not separated in Fig. 1. liths, respectively, and rocks of this composition may The term noritic anorthosite is used for rocks with prove to be more common than originally though t. color index more than 10 and less than 22t (Budding- The more leucocratic norites from the San Marcos ton, 1939, p. 19), and plagioclase more calcic than gabbro of the Southern California batholith (Larsen, An50. Those rocks with plagioclase more sodic than 1948) are also quite similar. All have similar textures An50 are called hypersthene diorites. The entire and appear to be early relative to their immediate series is of interest because of its unusual composition surroundings. Modes, chemical analyses, and norms considering its position in the composite Sierra of two hypersthene diorites are given in Table 1. Nevada batholith, its layered structures, and its con- Each body in the Eagle Lake sequence is struc- sistent trend of mineral compositions with decreasing turally an individual intrusion with foliation which age. generally parallels contacts and truncates structures The quartz diorite and minor granodiorite bodies in older adjacent bodies. Combed Willow Lake-type listed as numbers 3 and 4 above intrude the noritic layering (Taubeneck and Poldervaart, 1960) lies rocks. They are shown as younger intrusive rocks in along many intrusive contacts within the complex, Fig. 1. Many of the quartz diorite bodies have hypers- and mayor may not be broken and deformed by thene grains rimmed with olive or brownish olive later magmatic movements. Cumulative layering 1 Present address: California Institute of Technology, Jet has been found in large blocky inclusions in younger Propulsion Laboratory, Pasadena, California. noritic rocks. The foliate and lineate fabrics of indi- 62 SIERRA NEVADAN ANORTHOSITES 63 MORAINE NORITIC SEQUENCE I~ \ ~ x I OLDER INTRUSIVE ROCKS CONTACT, DASHED WHERE ~// APPROXIMATE »> FAULT ROAD ~ ( - TRAIL "- _/ STATE OF CALIFORNIA N SHOWING LOCATION OF o 5000 10000 FALLEN LEAF LAKE -----. FEET 15' QUADRANGLE r GEOLOGIC MAP SHOWING LOCATION OF EAGLE LAKE NORITIC SEQUENCE FIG. 1. Generalized geologic map of the northeast part of the Fallen Leaf Lake 15-minute quadrangle. TABLE 1. MODES, ANALYSES AND NORMS FOR Two SPECIMENS FROM THE EAGLE LAKE SEQUENCE Modes, Vol. % Analyses, WI. % Norms, WI. % 593 600 593 600 593 600 plagioclase 81.7 75.8 Si02 52.2 51.3 apatite 0.3 0.7 orthoclase 0.7 0.1 AJ,03 21.7 20.2 ilmenite 0.6 2.0 quartz none 0.4 Ti02 0.3 1.1 magnetite 4.9 5.6 hypersthene 9.5 8.4 Fe203 3.3 3.9 orthoclase 3.3 2.1 augite 2.7 1.7 FeO 5.2 4.3 plagioclase 77 .1 70.8 hornblende 1.0 2.5 MnO nd 0.12 diopside 2.3 5.3 biotite 0.3 1.1 MgO 3.8 3.6 hypersthene 11.1 12.7 magnetite 4.0 4.9 CaO 8.8 8.6 quartz 1.2 0.8 apatite 0.1 0.2 Na20 4.0 4.35 calcite, chlorite none 5.0 K20 0.5 0.35 normative An 50 47 H2O+ nd 1.4 normative En 66 70 modal An 45 45 p,Os 0.1 0.30 modal En 60 68 CO, nd 0.54 density 2.88 2.84 Total 99.9 100.06 Analysis 593 by Albert G. Loomis. Analysis 600 by E. L. P. Mercy. 64 ALDEN A. LOOMIS vidual plutons are not directly related to either type of layering. Most of the plagioclase and orthopyroxene in the plutons crystallized before development of foliation. The bulk of the augite present crystallized after the bodies had been emplaced and the fabrics had been developed. Subhedral and euhedral plagioclase tablets define a flow foliation (exclusive of the layered structures) and c-axes of subhedral bronzite or hypersthene grains define a lineation in the foliation plane. Augite occurs both as discrete crystals or as jackets surrounding the orthopyroxenes. Much augite is poikilitic, enclosing several pyroxene and plagio- clase grains. Olive-green to brownish-olive horn- FIG. 3. Irregular mode of amphibolization in hypersthene blende has jackted and partially replaced the pyrox- diorite near contact with another hypersthene diorite at lower left. enes and plagioclase. Hornblende rims are commonly continuous around several grains which are aligned in magnetite is late; it replaces and is interstitial to the fabric; therefore, most, if not all, of the horn- earlier phases. The small colorless grains in the ore blende crystallized after the internal movements of are apatite. The amount of magnetite varies very the bodies had ceased. Amounts of hornblende vary; little within the complex, generally forming 4 to 5 amphibolization is described below. Magnetite volume per cent of the rocks. The amount of horn- crystallized subsequently or nearly contemporane- blende is quite variable, however, as is the presence of ously with hornblende. It replaces pyroxenes and deuteric alteration. The rocks are amphibolized par- hornblende and is interstitial to plagioclase, py- ticularly along contacts and fractures, as in Fig. 3. roxenes, and hornblende. Late deuteric alterations Figure 4 is a cumulative mode diagram of 14 speci- are local. Common minerals include chlorite, actino- mens in varying states of alteration. The extent of lite, biotite, and calcite. actual replacement of plagioclase by hornblende, not Figure 2 illustrates a common texture in the noritic only the reaction of pyroxene and plagioclase to anorthosite. Single hornblende grains surround hornblende, is illustrated by the changes from the pyroxenes and replace plagioclase. The bulk of the fresh rock No. 593 to the amphibolized No. 622. Al- though the volume percentage of hornblende in- .,c: .,o Q. W ::;; ::J _j o > 60~~VO~'~~6~2~7~7~7~6~5~4'A'6~2~3~6~2~2~6~22'-~'6~2~2-L_'6okoO-~~6~5~6~6"'5~4'"5~94~593 SPECIMEN NUMBER FIG. 2. Noritic anorthosite specimen 622. Olive-green hornblende FIG. 4. Cumulative modes of 14 noritic anorthosites and hyper- rims pyroxenes. Magnetite, commonly with apatite inclusions, is sthene diorites which had very similar modes prior to amphiboliza- late, fills voids, and replaces pre-existing mafic grains. tion and deuteric uralite, quartz and biotite. SIERRA NEVADAN ANORTHOSITES 65 creases from a minimum of zero per cent to a maxi- mum of 13 per cent, the volume of the pyroxene shows a complementary decrease of 4 per cent or less. The volume now occupied by green hornblende was originally occupied mostly by plagioclase. The pyroxenes remaining after the amphibolization and introduction of late magnetite were uralitized locally, and some deuteric biotite and quartz formed. These alterations took place mainly near contacts with older plutons and where the rocks were intruded by the surrounding quartz diorites. Iron was more strongly concentrated during the crystallization of anyone body than during the entire intrusive sequence.
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